In currently used printers, there are several printing methods such as an inkjet printing method, a thermal transfer method, and a laser printing method in which a conventional copy machine is electronized. In these methods, ink, ink-ribbons, and toner are used to transfer data to a paper. Therefore when printing work has finished, used ink, ink cartridges, and ink-ribbons are discarded as industry wastes. For photograph printing, it is necessary to use a special paper.
But in the case of a thermal printer, though a special paper is necessary, it is not necessary to discard ink, ink cartridges and ink-ribbons. The thermal printing method has been adopted in facsimiles and bar code printing machines. However, the thermal printing method has too many problems to achieve a photograph-quality color image. At present, the thermal printing method cannot achieve photograph quality.
The largest technical problem is that a thermal head cannot control each of three-color layers independently by using its heat control, so that a color image of rich color gradation has not been achieved. However, there is only one product using the thermal printing method, which was released in 1996 and adopted a direct thermal recording method (TA method) based on a technical concept described in the Japanese patent document 1 below.
In the TA method, three thermal color-forming layers of cyan, magenta, and yellow are stacked up in order on a support sheet and a top layer is a thermal-protective layer. The yellow and magenta color-forming layers include diazonium salt compound and a coupler as a color developing material, which can be fixed with ultraviolet light. The cyan color-forming layer uses a dye precursor and organic acid as a color developing material, which does not need fixation. Microcapsules in each of the three layers have different thermal sensitivities and different ultraviolet sensitivities. These microcapsules are processed through five steps, using different thermal energy and different-wavelength ultraviolet light. Thus they are developed and fixed through the five steps to make a full-color print.
The five steps in the TA method are as follows: (1) forming a yellow image by using yellow information with low thermal energy, (2) radiating 419 nm ultraviolet light to an entire image to fix the yellow image, (3) forming a magenta image by using medium thermal energy (the developed yellow image is not affected by the medium thermal energy), (4) radiating 365 nm ultraviolet light to an entire image to fix the magenta image, and (5) forming a cyan image by using high thermal energy.
According to Japanese patent document 2 below, another method is described that uses the combination of three steps of different pressures and three steps of different temperatures, resulting in the chemical reaction between diazonium salt compound inside capsules and coupler outside capsules.    Japanese patent document 1: Japanese Unexamined Patent Application No. 61-40192    Japanese patent document 2: Japanese Unexamined Patent Application No. 11-170692